1. Field of the Invention
The present invention relates to an electronic flash apparatus capable of high-speed consecutive flash emissions.
2. Related Background Art
FIG. 7 shows a conventional example of the electronic flash apparatus. Between a power supply line l.sub.11 and a ground line l.sub.12 there are serially connected a flash discharge tube Xe such as a xenon discharge tube and a main thyristor SCR11, and the trigger electrode and the cathode of the flash discharge tube Xe are connected to the secondary coil of a trigger transformer T, which constitutes a trigger circuit TC in combination with a trigger capacitor C11. To said trigger circuit TC there is connected a trigger thyristor SCR12 which is rendered conductive by a trigger signal supplied to a gate thereof through a line l.sub.13. Also there is provided a known current diverting circuit including a current diverting capacitor C12 and a current diverting thyristor SCR13.
When the trigger thyristor SCR12 is rendered conductive in a state in which an unrepresented main capacitor, a trigger capacitor C11 and a current diverting capacitor C12 are charged, a high voltage is generated in the secondary coil of the trigger transformer T and is applied between the trigger electrode and the cathode of the flash discharge tube Xe, whereby the flash discharge tube Xe starts discharge and light emission by the electric charge in said unrepresented main capacitor, but sufficient light emission is not obtained at this point since the main thyristor SCR11 is still non-conductive.
In this state the potential of the junction point of anode of the main thyristor SCR11, cathode of the flash discharge tube Xe, resistor R11 and current-diverting capacitor C12 (hereinafter called point K) is elevated. The potential increase of the point K results in a potential increase of the other terminal of the current-diverting capacitor C12 (hereinafter called point L), and further in a potential increase at the other terminal of a capacitor C13, namely the junction between the capacitor C13 and a resistor R12, whereby said potential increase is transmitted through said resistor R12 and a line l.sub.14 to the gate of the main thyristor SCR11. The main thyristor SCR11 is thus rendered conductive, and the flash discharge tube Xe starts main light emission.
At the same time, an unrepresented light metering system starts light metering, and a flash stop signal is released to a line l.sub.15 when a predetermined amount of flash is reached. In response a current-diverting thyristor SCR13 is rendered conductive, thus inversely biasing the main thyristor SCR11 and terminating the flash emission of the flash discharge tube Xe in the known manner.
The amount of each light emission is determined by the time from the supply of a flash start signal to the line l.sub.13 to the supply of the flash stop signal to the line l.sub.15. Thus the amount of light emitted from the flash discharge tube Xe can be decreased or increased by extending or shortening said time.
Consecutive flash emissions with limited amount of light each time can be obtained by reducing the interval of flash start signals, but said interval cannot be shortened excessively because of the following reason.
The trigger capacitor C11 constituting the trigger circuit TC, has a relatively large charging resistor R13 and cannot therefore be charged in time if the interval of flash emissions is reduced. Thus, even when the thyristor SCR12 becomes conductive, the trigger voltage cannot be applied to the flash discharge tube Xe since the trigger capacitor C11 is not charged. The resistance of the charging resistor R13 for the trigger capacitor C11 has to be decreased in order to reduce the interval of the flash emissions, but said resistance has a lower limit in relation to the holding current of the trigger thyristor SCR12. Consequently such conventional circuitry can control the amount of light emission each time, but cannot achieve consecutive flash emissions at a high speed.